The invention relates to a variable valve drive for a cam-actuated lift valve of an internal combustion engine which is loaded by a closing pressure against the direction of opening, with a hydraulic system comprising a hydraulic power generating device acting against the closing force on the lift valve with a control piston arranged longitudinally displaceable in a cylinder and adjacent to a hydraulic control chamber, with a stroke of the lift valve produced by an actuating cam being at least reducible by gradually removing hydraulic liquid from the control chamber by means of a control valve.
A variable valve drive for an internal combustion engine with two intake valves per cylinder is known from U.S. Pat. No. 5,839,400. By relieving the pressure of a chamber disposed between the tappet and the intake valve it is possible to achieve an uncoupling of the stroke movement of the intake valve from the mechanical stroke curve as predetermined by the intake cam. Such a system is known as a xe2x80x9clost motionxe2x80x9d system. Such xe2x80x9clost motionxe2x80x9d systems are characterized in that the stroke curve which is predetermined by the shape of the actuating cam can only be reduced, but in no way increased. No additional strokes are thus possible.
The U.S. Pat. No. 5,127,375 A for example also describes such a valve drive. The disadvantageous aspect is that in this case too there is no active pressurization within the terms of a hydraulic lifting apparatus and thus no multiple opening of the lift valve per work cycle is possible in a hydraulic manner.
U.S. Pat. No. 5,216,988 A describes a valve actuating device where the pressure generation and the pressure transmission occurs in the bucket tappet. Air bubbles can be removed from the system with a scavenging pump connected in the interior of the bucket tappet and a run-off control valve on the run-off side.
U.S. Pat. No. 5,005,540 A describes a valve control device with a hydraulic bucket tappet disposed between the. cam and lift valve. An admission pressure is produced via an external pump in the hydraulic bucket tappet. The gradual depressurization of the pressure chamber of the bucket tappet occurs via a solenoid valve. An active hydraulic lifting of the valve is also not possible in this case.
A variable valve drive for a lift valve is known from DE 43 17 607 A1 with which a hydraulic additional stroke can be produced during the mechanical lifting phase by a cam. A hydraulic lift is only possible in the known valve drive as long as the pressure line with the pressure conduit, which pressure line is fixed to the housing, overlaps with the power generating device which is formed by a bucket tappet. While the base circle of the cam attacks the bucket tappet, the supply of pressure medium to the bucket tappet is interrupted. The possibility of the hydraulic activation of the lift valve is thus limited to a very short period from a geometrical viewpoint. A reduction of the valve lift within the terms of a xe2x80x9clost motionxe2x80x9d system is not provided. The valve lift and the valve control times can thus only be influenced to a very low extent.
It is the object of the invention to avoid such disadvantages and to arrange the valve lift and valve opening in the freest possible way in a valve drive of the kind mentioned above.
This is achieved in accordance with the invention in such a way that the pressure chamber is connectable with a high-pressure level preferably by means of the control valve. In this way it is possible, in addition to the xe2x80x9clost motionxe2x80x9d function, to also achieve an active hydraulic valve actuation. It is thus possible to realize both control times and valve lifts which are situated below the cam-induced mechanical stroke curve of the lifting valve as well as such which are situated above the cam-induced mechanical movement of the lift valve.
In order to realize the desired functions of xe2x80x9clost motionxe2x80x9d and xe2x80x9cvariable valve actuationxe2x80x9d it is advantageous when the control valve is a 3/3-way valve which is connected to a control line connected to the control chamber, to a high-pressure line as well as a medium-pressure line. In a particularly compact embodiment it is provided that the control valve is arranged as a control slide valve.
It is provided for in an exceptionally preferred embodiment of the invention that a medium-pressure feed line opens into the control chamber, with the medium-pressure feed line preferably comprising a first non-return valve opening in the direction of the control chamber and preferably the medium-pressure line being arranged as a depressurization line for the control chamber. As a result, a hydraulic valve play compensation can be achieved at a quasi static resting point of the dragging lever. The control valve is situated in a medium position in which the connection between the control chamber and the high-pressure line on the one hand and the medium-pressure line on the other hand is interrupted. The xe2x80x9clost motionxe2x80x9d function can be achieved by gradually removing the hydraulic liquid from the control chamber.
In order to ensure an even pressure in the high-pressure line, it is advantageous when the high-pressure line is connected via a first throttle device with a high-pressure reservoir. The high-pressure reservoir can be formed by the distributor line of a fuel injection system. A constant pressure in the medium-pressure line is ensured when the medium-pressure line opens via a second throttle device into a supply tank for the hydraulic liquid.
In a particularly preferable embodiment of the invention it is provided that the high-pressure line and the medium-pressure line are connected with each other via a third throttle device. The throttle devices can be configured as control valves in order to enable a change of the pressure in the high-pressure or medium-pressure line over a wide range. The first, second and third throttle devices form a cascade control which allows operating the variable valve drive with the lowest possible amount of energy input because only such an amount of fuel is taken from the high-pressure reservoir as is momentarily needed by the variable valve drive. In order to adjust the total conveyed quantity, it is provided further that the medium-pressure feed line, is connected downstream of a fore-pump via a fourth throttle device and a high-pressure pump with the pressure line opening into the high-pressure reservoir. The required total conveyed quantity for injection, variable valve drive as well as a control reserve are defined by the fourth throttle device which is arranged as a control valve.
According to a further preferred embodiment of the invention, it is provided that the medium-pressure line is connected via a hydraulic pressure intensifying device with the high-pressure reservoir. As a result of the hydraulic pressure intensifying device, it is possible to use the gradually removed quantity from the pressure chamber during a xe2x80x9clost motionxe2x80x9d function of the valve drive in order to fill the pressure reservoir at a low pressure level (medium pressure). Preferably it is provided that the pressure intensifying device is provided with a differential piston adjacent to a working chamber and a pressure chamber, with the medium-pressure line opening into the working chamber and with a pressure line starting out from the pressure chamber which leads to a high-pressure reservoir. At least a partial stroke of a lifting valve is thus used in order to achieve a pumping effect. In order to enable the filling of the high-pressure reservoir by the pumping effect of the lifting valve at a low pressure level, a second non-return valve which opens in the direction of the working chamber is provided on the one hand in the medium-pressure line and a third non-return valve which opens in the direction towards the high-pressure reservoir is provided on the other hand in the pressure line between the pressure chamber and the high-pressure reservoir. In order to feed the pressure chamber of the pressure intensifying device, the same is connected via a feed line with the medium-pressure line, with a fourth non-return valve which opens in the direction towards the pressure chamber being arranged in the feed line.
If, in the case of multi-valve engines for example, only one intake and one exhaust valve are actuated for the gas change in partial-load operation following a port shut-off, the non-actuated valves can be used for filling the high-pressure reservoir. This is especially possible when the camshaft comprising the actuating cams revolves with the crankshaft speed.
It is especially advantageous when a single pressure intensifier and a single high-pressure reservoir are provided for at least for one group of lift valves. Preferably, one medium-pressure line per lift valve is connected with the pressure intensifier and preferably one high-pressure line per lift valve is connected with the high-pressure reservoir. The control of the pressure intensifying device occurs discontinuously by means of a 2/2-way valve which is arranged in a depressurization line which is connected with the working chamber. In order to achieve an individual control of the strokes of the lift valves it is advantageous when a control valve is provided for each lift valve.
A particularly simple and energy-saving variable valve drive can be achieved when the actuating cam acts upon the lift valve via a dragging lever. Preferably, the hydraulic power force generating device is arranged in the zone of the dragging lever bearing, and the bearing point of the dragging lever bearing is adjustable by the power generating device. Moreover, the hydraulic power generating device can also be used in valve drives with rocker levers.
Alternatively, the actuating cam can also act upon the lift valve via a bucket tappet, with the bucket tappet preferably being arranged as a hydraulic power generating device.
The hydraulic system can be connected with a lubricating oil circuit or with a fuel system such as a storage injection system of the internal combustion engine.